There exist audio coding systems for parametric spatial audio coding. For example, MPEG Surround describes a system for parametric spatial coding of multichannel audio. MPEG SAOC (Spatial Audio Object Coding) describes a system for parametric coding of audio objects.
On an encoder side these systems typically downmix the channels/objects into a downmix, which typically is a mono (one channel) or a stereo (two channels) downmix, and extract side information describing the properties of the channels/objects by means of parameters like level differences and cross-correlation. The downmix and the side information are then encoded and sent to a decoder side. At the decoder side, the channels/objects are reconstructed, i.e. approximated, from the downmix under control of the parameters of the side information.
A drawback of these systems is that the reconstruction is typically mathematically complex and often has to rely on assumptions about properties of the audio content that is not explicitly described by the parameters sent as side information. Such assumptions may for example be that the channels/objects are treated as uncorrelated unless a cross-correlation parameter is sent, or that the downmix of the channels/objects is generated in a specific way.
In addition to the above, coding efficiency emerges as a key design factor in applications intended for audio distribution, including both network broadcasting and one-to-one file transmission. Coding efficiency is of some relevance also to keep file sizes and required memory limited, at least in non-professional products.
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